1. Field of the Invention
The present invention relates generally to an anti-lock brake control system for a motor vehicle in which driving wheels are driven by a prime mover such as an internal combustion engine by way of a torque transmitting member such as an axle shaft, a drive shaft or the like to which a torsion torque is applied. In more particular, the invention is concerned with the anti-lock brake control system which is capable of controlling brake forces applied to the wheels in such manner that the vehicle can stop with a short stopping distance upon braking, while ensuring a high stability for the operation of the brake system as well as high controllability of the motor vehicle. The invention is further concerned with a brake force control method which can be carried out by a microcomputer or the like.
2. Description of Related Art
In general, in the anti-lock brake control system for the motor vehicle, the trend of wheels being locked is detected on the basis of the result of comparison between the wheel speed (given, for example, by a rotation number of the wheel per minute or rpm) and an estimated speed of the motor vehicle or on the basis of deceleration of the wheel or the like. In that case, braking hydraulic pressure applied to the wheel is so regulated that magnitude of skid of the wheel relative to the road surface is maintained at a value close to a region in which friction between the wheel and the road surface assumes a peak value, with a view to shortening the stopping distance of the motor vehicle, while ensuring stability of the vehicle body and enhancing the manipulatability or driving performance of the motor vehicle. By way of example, in the conventional anti-lock brake control system known heretofore, decision is made to the effect that the wheel of the motor vehicle tends to be locked when behavior of the wheel such as a slip thereof which represents a sink of the wheel speed relative to the estimated vehicle speed attains a predetermined threshold value, whereupon the braking hydraulic pressure applied to the wheel is lowered under control.
As is apparent from the above, in the conventional anti-lock brake control system, the braking hydraulic pressure is controlled on the basis of the wheel speed and the wheel acceleration. Accordingly, when the brake force is applied steeply, the driving wheels coupled to an internal combustion engine (hereinafter also referred to simply as the engine) by way of the wheel drive shaft(s), i.e., the driving wheels are decelerated at a high rate. On the other hand, the engine which exhibits a large inertia is caused to decelerate only slowly when compared with the driving wheels. As a consequence, torsion of a great magnitude takes place in the drive shaft coupling the driving wheels and the engine. Under the influence of this torsion, the wheels are caused to decelerate and accelerate respectively. In other words, vibration occurs in the driving wheels.
Consequently, the braking hydraulic pressure is lowered in an effort to suppress the vibration of the driving wheels as brought about by the torsion taking place in the drive shaft regardless of the condition of the road on which the motor vehicle is running, as a result the stopping distance becomes longer. Furthermore, in the case where the motor vehicle is running on a road having a surface of small coefficient of friction (hereinafter also referred to as the frictional coefficient), the vibration of the driving wheel due to the torsion of the drive shaft may erroneously be decided that the vibration is ascribable to a rough road condition. In that case, a higher braking pressure exceeding the reaction force exerted by the road surface may be unwontedly sustained, whereby the trend of the wheels being locked is promoted, incurring instability in the running performance of the motor vehicle as well as degradation in the controllability (i.e., stable manipulatability) thereof. For these reasons, with the anti-lock brake control system known heretofore, there may rise such situation that optimal brake force which conforms with the running states of the motor vehicle and the road condition can not always be ensured.
As the measures for coping with the problems of the conventional anti-lock brake control system, the inventors of the present application have already proposed an improved anti-lock brake control system in which a torsion torque occurring between the engine and the driving wheel(s) operatively connected to the engine via a drive shaft is detected, whereon the wheel acceleration is corrected in consideration of the detected torsion torque to thereby determine a corrected acceleration to be employed as a control parameter for the anti-lock brake control as is disclosed in Japanese Unexamined Patent Application Publication No. 296693/1994 (JP-A-6-296693). The corrected acceleration can be arithmetically determined as follows:
When a torsion is applied to the drive shaft for the wheels, the equation of motion of the wheel in which the torsion is taken into consideration can be expressed as follows: EQU Iw.multidot.(d.omega./dt)=.mu..multidot.W.multidot.r-Tb-Tt (1)
where
Iw represents the moment of inertia of the wheel, PA1 .omega. represents an angular velocity of the wheel, PA1 Tt represents a torsion torque, PA1 .mu. represents a coefficient of friction of a road surface, PA1 W represents a load imposed on the wheel, PA1 r represents a radius of the wheel, and PA1 Tb represents a brake torque.
Relation between the wheel angular velocity .omega. and the wheel acceleration Gw can be expressed as follows: EQU Gw=Kr.multidot.(d.omega./dt) (2)
where Kr represents a constant. From the expressions (1) and (2), the following expression (3) can be derived. EQU Gc=Gw+(Kr/Iw).multidot.Tt (3)
Thus, corrected acceleration Gc can be determined on the basis of the wheel acceleration Gw and the torsion torque Tt in accordance with the above expression (3).
Furthermore, from the expressions (1) and (3), the corrected acceleration Gc can also be expressed as follows: EQU Gc=(Kr/Iw).multidot.(.mu..multidot.W.multidot.r-Tb) (4)
To say in another way, a relation between the tire torque .mu..multidot.W.multidot.r determined by the frictional coefficient .mu. of the road surface, the reaction force .mu..multidot.W exerted by the road surface in response to the wheel load W and the wheel radius r on one hand and the brake torque Tb generated by the braking hydraulic pressure can be determined on the basis of the corrected acceleration Gc.
More specifically, it can be detected on the basis of the corrected acceleration Gc whether or not the tire torque exceeds the brake torque or how much difference exists between the tire torque and the brake torque. In other words, the relation between the reaction force of the road surface and the brake force can be determined on the basis of the corrected acceleration Gc. When it is indicated by the corrected acceleration Gc that the brake force becomes smaller than the reaction force of the road surface, lowering of the brake force is inhibited, whereas when the wheel acceleration is sufficiently high, the brake force is increased.
In this manner, by using for the anti-lock brake control the corrected acceleration derived by correcting the wheel acceleration in consideration of the torsion torque, brake force optimal for the reaction force exerted by the road surface can be applied because of possibility of regulating the braking hydraulic pressure in dependence on the behaviors which the wheel exhibits under the influence of the torsion torque.
Parenthetically, the torsion torque can be arithmetically determined on the basis of the detected rotation speed (rpm) of the engine or the drive shaft without difficulty, as readily appreciated by those skill in the art.
In the above-mentioned anti-lock brake control system proposed in precedence, the anti-lock brake control is affected by the conditions for the timing to start decreasing or increasing of the braking hydraulic pressure or for the timing to stop decreasing or increasing of the braking hydraulic pressure. More specifically, the anti-lock brake control is effectuated by making a decision on the basis of the corrected acceleration whether the brake force exceeds the reaction force of the road surface or alternatively the brake force becomes smaller than the reaction force of the road surface. Consequently, the corrected acceleration and hence the torsion torque can be utilized only for determining the timing to stop decreasing the braking hydraulic pressure or alternatively the timing to start increasing the braking hydraulic pressure. For the other controls involved in the anti-lock brake control, the torsion torque can not be employed positively.
In an attempt to detect the torsion torque by using an inexpensive means, the rotation speed (rpm) of the prime mover such as the engine is detected, whereon the torsion torque is arithmetically determined on the basis of the rate of change of the rotation speed. In practical applications, the rotation speed of the engine is determined on the basis of the pulses derived from a crank angle sensor installed ordinarily for detecting the crank angle of the engine. In this conjunction, it is noted that the number of the rotation angle pulses outputted from the crank angle sensor which depends on the rotation speed (rpm) of the engine is remarkably smaller than the number of rotation angle pulses outputted from a wheel speed sensor which serves as a wheel speed detecting means. Consequently, when compared with the information concerning the wheel speed, a lot of time is taken for making available the engine rotation number and hence the torque information. As a result of this, there may arise such situation that a delay is involved in making decision concerning the stoppage of reduction of the braking hydraulic pressure on the basis of the corrected acceleration, which may naturally lead to a delay in the timing to stop decreasing of the braking hydraulic pressure, as a result of which the brake application pressure lowers excessively. In that case, the brake force will become inadequate when the motor vehicle is running on a road having a road surface of high frictional coefficient.